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Neale, your readings of the resin accuracy are interesting. Seves thou is 7*0.0254=0.18mm which is a bit more than I'd expect (though by no means a showstopper). Did you see the readings for the expoxy 'straightness' I took in my build log?

Towards the end of that graph there is a large negative deviation. It turns out this was not actually the case - the bearing block was rubbing on the aluminium epoxy barrier which pushed it out of line for the last 100mm or so. Either way, the deviation is very small...

Here is the corresponding error plot for my rail (as I have only managed to fix down one rail so far. Once visitors have left...)

I have converted to mm for ease of comparison. We used different resins, although I'm inclined to dismiss that one as a significant factor - they are both at the runny end of the epoxy viscosity scale. However, I was working at an ambient temperature of around 10C at the time of pouring and although I had kept the resin indoors for a few days beforehand, it was a thin film sitting on steel and will have been down to garage ambient almost immediately. That won't have done wonders for viscosity and maybe it didn't flow as well. I think my rail section error was also rather greater to start with, and as my errors are roughly aligned with the initial error (the bends in the rail) I wonder if these are linked. Better flow where the epoxy is deeper?
There is also the measurement technique to consider. I don't have a surface plate big enough to be useful - mine's a sheet of thick plate glass about 300mm square - so I used a level at a series of points, combining level scale readings with feeler gauges to get a "difference in height" value. I dropped all these into a spreadsheet to get the graph shown here. It wouldn't be difficult to calculate shim values to bring the rail into line. Is it worth it? Assuming, obviously, that the bed is finished to the rails, then a hump in the middle wouldn't be a problem as the workpiece would tend to droop over it, being clamped at the edges. However, it is more likely to bridge a hollow in the bed. And I'm quite certain that my sophisticated woodworking techniques would dramatically fail with an error of 0.18mm in depth over a 1.8m length...

This post is probably going to open me up to derision from the practical community ("that's close enough, why are you bothering about 7 thou?") and the more academic ("your sums are wrong and your measurements are suspect!") but I'm feeling brave so here's my theory.

I believe that the dip in the surface of my epoxy is due to shrinkage, and there is a lesson to be drawn from that in how you should use epoxy. This evening, I measured, as closely as my measuring kit would allow, the depth of the epoxy bed at the end and the centre of my X rail. If I assume that the difference in level is due to shrinkage, the difference in levels of the hardened epoxy corresponds to a shrinkage amount of approximately 10%. That is, after setting, the epoxy bed will end up about 90% of the depth of the liquid resin. That's using the resin as per the first post of this thread. I suspect that a nominal 10% linear shrinkage will not lead to an exact 10% reduction in depth as there will be other smaller effects due to shrinkage across as well as down through the epoxy but I'm ignoring three-dimensional effects for the moment. I'm happy to go through how I did my calculations if asked, but for the moment I'll skip directly to my conclusions.

An application of this epoxy will lead to a reduction in "depth" errors by an amount dependent on the shrinkage factor of the epoxy used. In my case, for example, I started with a dip in the rail surface of about 1.7mm; after epoxy treatment I ended up with a dip of about 0.18mm, a reduction of about 90%. If you think about it, this will happen whatever the depth of epoxy used (unless there is some other effect associated with shrinkage). However, if I now used a second layer of epoxy on top of the first, then I would reduce this dip by about 90% again, and I would expect to see an dip of about 0.02mm after that. In other words, for a given total depth of epoxy, you would do better to use it in two thin layers than one thick layer.

Does this also explain the effect that some people have seen where there is a noticeable shrinkage effect at rail/bridge joins - if you are not careful to get the bridge at the same height as the rail, then there will be a different amount of absolute shrinkage either side of the join and that might affect the resulting levels?

Jonathan's measured error curves are not quite in agreement with this principle, but his measurement technique is dependent both on rail height variation and rail twist, and once we are getting down to small numbers the measurement technique becomes important. However, it is interesting (but possibly coincidental) that his initial error was reduced from about +-0.3mm to about +-0.03mm. That's the kind of reduction that my thinking would have suggested although it doesn't prove anything due to, as I say, the different measurement technique.

If I had thought about it beforehand, then all this would have been blindingly obvious but then, I would not have expected quite as much shrinkage. Looks like epoxy is great, but it reduces rather than removes build alignment errors and the more accurately you can build the initial structure, the better the result after using epoxy. Shimming, on the other hand, should be able to take out arbitrary errors, although it's going to be much more difficult to measure and remove rail twist.

eHowever, if I now used a second layer of epoxy on top of the first, then I would reduce this dip by about 90% again, and I would expect to see an dip of about 0.02mm after that. In other words, for a given total depth of epoxy, you would do better to use it in two thin layers than one thick layer.

I'm happy to go through how I did my calculations if asked, but for the moment I'll skip directly to my conclusions.

I think your theory is worth exploring and (you guessed it) I'd like to confirm that your measurement method is sound since we don't want to be victims of correlation not implying causation.

Have you measured the shrinkage of your resin directly, since clearly if the resin shrinkage is far from 10% the rest of your analysis is potentially invalid (though not necessarily invalid, since it could still exist as a small effect masked by something else)? If you've not measured or got the data elsewhere, then perhaps pour some into a fairly thin tube, mark the level and when it sets measure the change, or think of a better method as I expect there will be one.

The west system resin datasheet says it 'does not shrink after curing', but does that mean it doesn't shrink during curing? I guess not else they'd surely specify. Adding thinners does increase the shrinkage, so one should be careful when selecting the resin to ensure it doesn't contain them as that option might seem attractive to reduce the viscosity.

Please could you elaborate on your measurement method to get the "height error from horizontal plane"? Did you add the feeler gauge under one end of the level until the level read zero, then note down the thickness of the gauge, or have I completely missed the point? If it's the former then the measurement is an angle from the horizontal plane (or strictly speaking the Earth's center of mass, but lets not worry about the earth curvature error), not height deviation without more careful interpretation of the readings.

Originally Posted by Neale

Jonathan's measured error curves are not quite in agreement with this principle, but his measurement technique is dependent both on rail height variation and rail twist, and once we are getting down to small numbers the measurement technique becomes important.

I could have indicated from the surface plate to the resin, or from the rail to surface plate at two distances to separate the readings of linear and angular error ... but time was not on my side.

Originally Posted by Neale

However, it is interesting (but possibly coincidental) that his initial error was reduced from about +-0.3mm to about +-0.03mm. That's the kind of reduction that my thinking would have suggested although it doesn't prove anything due to, as I say, the different measurement technique.

I'm not sure where you got the +-0.3mm from, but from the graph I posted in the thread it was more like +-0.5mm, then 0.3mm after the failed compensation method using a machined strip. However the graph I posted is a combination of the height and twist errors, so we can't actually conclude from this what the height error was.

We could gain confidence in your theory about the resin shrinkage by measuring if the height error follows the original profile, just with a smaller magnitude. My readings do not seem to follow the original profile, but as mentioned already the readings are a combination of the rotational and linear error, so we can't draw any conclusion from this.

Maybe I should try a simple test using my surface plate - create three channels, one directly on the surface plate with the surface plate horizontal (the 'control' experiment) and one with the surface plate tilted so the resin is significantly deeper at one end. Measure the linearity of both and see if there's a discernible difference between the samples.

If you have read carefully my build, and implemented the technique i suggested that would not have happened. Epoxy shrinks and that is life.

Read from post #77 on, or jump directly at the solution. i think i should write an epoxy guide and make it sticky. As i already paid the price to learn/~6kg of wasted expensive epoxy, 2 pours wasted for learning purposes , so no need for others to repeat the mistakes.

Your epoxy shrunk at the bridge, cause the bridge shrunk lengthwise and sucked the epoxy from the center of the rail. Thats all.
I confirmed that in my build by calculating the given shrinkage of the epoxy, calculating the volume, length and so and it gave me as a result the exact shrinkage that happened.

One thing more. The glass transition temp of WS is 50C. So take care for direct sunlight in summer not to fall onto the epoxy, especially with heavy gantry. It will f^^ck up. I made some experiments and it becomes like a chewing gum quite fast. Faster than i liked.

The Following User Says Thank You to Boyan Silyavski For This Useful Post:

If you have read carefully my build, and implemented the technique i suggested that would not have happened. Epoxy shrinks and that is life.

Read from post #77 on, or jump directly at the solution. i think i should write an epoxy guide and make it sticky. As i already paid the price to learn/~6kg of wasted expensive epoxy, 2 pours wasted for learning purposes , so no need for others to repeat the mistakes.

Your epoxy shrunk at the bridge, cause the bridge shrunk lengthwise and sucked the epoxy from the center of the rail. Thats all.
I confirmed that in my build by calculating the given shrinkage of the epoxy, calculating the volume, length and so and it gave me as a result the exact shrinkage that happened.

One thing more. The glass transition temp of WS is 50C. So take care for direct sunlight in summer not to fall onto the epoxy, especially with heavy gantry. It will f^^ck up. I made some experiments and it becomes like a chewing gum quite fast. Faster than i liked.